Fix operator handling with custom ops that start with /, and with ...

[alex-pinkus]

• Fix handling of custom operators when a slash was already consumed:

  • This logic was not working properly when the whitespace handler attempted to consume a slash already. To handle that, we give the eat_operator function some more information about the surrounding context of the possible prior character that has been eaten.

• Don't allow a three-dot operator after a newline:

  • The Swift compiler actually doesn't allow this with any whitespace around it, but that's more annoying to accomplish. Instead, we simply hard-code the three dot operator as "invalid" if we've already crossed over an implicit semicolon boundary. Once we emit the _semi, the three-dot operator will no longer be legal anymore, so it will be missed on the next pass as well.
  • We don't add a test for this because tests for error conditions are somewhat brittle. If there are a lot more of this sort of thing, it might be worth investing in some custom infrastructure to test that certain snippets fail without having to assert exactly the failing structure.